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Topic Name: Scientists have Shown that a Chunk of Hematite Can Conduct Electrons Under Certain Chemical Conditions
Category: Electrical
Research persons: Kevin Rosso
Location: Pacific Northwest National Laboratory, Department of Energy, United States
Details
If the Flintstones had electricity, their wires might have
been made of rock. New results in Science Express show that a chunk of hematite
can conduct electrons under certain chemical conditions. In addition, the
current causes some mineral surfaces to build up while others degrade. These
results with iron oxide might be important for water quality, soil evolution,
and environmental cleanup.
"Considering iron as an important nutrient, the finding could
help us understand how soils evolve from nutrient rich to nutrient poor," says
lead investigator Kevin Rosso, a chemist at the
Department of Energy's Pacific
Northwest National Laboratory. "And it has implications for other common
minerals such as pyrite and manganese oxides -- even particles in the
atmosphere."
Scientists have long known that electrons can travel through
some iron oxides when a voltage is applied, but they have assumed that electrons
stemming from chemical reactions alone won't move spontaneously through the
mineral's bulk. That long-standing assumption has caused chemists to treat
different faces of a hunk of mineral as independent entities that don’t
‘communicate’ with each other. New results, published online March 6, 2008 in
Science Express, suggest otherwise.
"Now we know reactions at different faces of these minerals
can couple together and yield behavior unique to semiconducting minerals," says
Rosso.
Minerals often exist as individual crystals in rocks at a
stream's bottom, where they keep busy reacting with the water flowing around
them. Understanding this chemistry is central to understanding how elements move
through sediments, maintaining good water quality, and cleaning up pollution. To
elicit the details, scientists study what effect acids and other forms of
chemicals have on mineral surfaces.
When Rosso and PNNL colleague Svetlana Yanina immersed a
cube-shaped hematite crystal in an acid solution in the absence of oxygen, they
expected all surfaces to degrade. But when the chemists examined the surfaces at
high magnification, they found one surface that didn’t. This surface grew
pyramid-like mounds rising from the top. "The whole crystal wants to dissolve,
thermodynamically," says Rosso. "So we didn't expect to see that growth."
No one had previously reported this buildup, so the team
modified their experiments to try to prevent the pyramids from growing. "In
fact, we spent a year trying to get rid of it," Rosso says.
One path to getting rid of something is to understand how it
got there in the first place, so they decided to explore how the pyramids
formed. The researchers performed atomic force, scanning electron and
transmission electron microscopy at the DOE's Environmental Molecular Sciences
Laboratory on the PNNL campus, as well as electrical potential measurements of
the individual surfaces.
Because hematite is a crystal of iron oxide, the sides and
the top (and bottom) are structurally different, and therefore have different
chemical properties. The team wondered if the iron being deposited on the top
came from iron dissolving from the sides, building up in solution, and then
redepositing.
To test this, they separated the six cube surfaces into
groups: They took two cubes, protected four sides from the solution on one, and
on the other, protected the top and bottom. The acidic solution chewed away the
unprotected surfaces, as expected. But the chemists didn't see any buildup on
the unprotected top and bottom faces and instead saw degradation. This indicated
the breakdown and buildup were not independent of each other.
"The hematite won't grow pyramids without that surface being
connected to the dissolving ones," says Rosso.
The required physical connection hinted at electron
conduction. Iron in solution, or Fe(II), contains one more electron than the
iron in the crystal, Fe(III). If Fe(II) landed on the top, it might react with
the surface, incorporate into the crystal and give up its electron. The electron
could then flow through the crystal to the sides, where an atom of Fe(III) could
pick up the electron and dissolve into the solution.
To prove this, the chemists connected the electron flow with
a wire. When they repeated the first experiment but connected the two cubes with
a dab of silver, the team restored the pyramid buildup. Additional experiments
allowed them to measure the electrical potential driving the current flow, which
came out to 200 millivolts -- about 6% of the power needed for a keychain LED
light, or about twice as much as in a nerve cell.
Note for Hematite
Hematite, also spelled hæmatite, is the mineral form of Iron(III) oxide (Fe2O3),
one of several iron oxides. Hematite crystallizes in the rhombohedral system,
and it has the same crystal structure as ilmenite and as corundum. Hematite and
ilmenite form a complete solid solution at temperatures above 950°C.
Hematite is a mineral, colored black to steel or silver-gray, brown to reddish
brown, or red. It is mined as the main ore of iron. Varieties include kidney
ore, martite (pseudomorphs after magnetite), iron rose and specularite (specular
hematite). While the forms of hematite vary, they all have a rust-red streak.
Hematite is harder than pure iron, but much more brittle.
Huge deposits of hematite are found in banded iron formations. Grey hematite is
typically found in places where there has been standing water or mineral hot
springs, such as those in Yellowstone. The mineral can precipitate out of water
and collect in layers at the bottom of a lake, spring, or other standing water.
Hematite can also occur without water, however, usually as the result of
volcanic activity.
Clay-sized hematite crystals can also occur as a secondary mineral formed by
weathering processes in soil, and along with other iron oxides or oxyhydroxides
such as goethite, is responsible for the red color of many tropical, ancient, or
otherwise highly weathered soils.
The name hematite is derived from the Greek word for blood (haima) because
hematite can be red, as in rouge, a powdered form of hematite. The color of
hematite lends it well in use as a pigment.
The magnetic structure of a-hematite was the subject of considerable discussion
and debate in the 1950s because it appeared to be ferromagnetic with a Curie
temperature of around 1000 K, but with an extremely tiny moment (0.002mB).
Adding to the surprise was a transition with a decrease in temperature at around
260 K to a phase with no net magnetic moment.
Dzialoshinksi and later Moriya showed that the system is essentially
antiferromagnetic but that the low symmetry of the cation sites allows
spin–orbit coupling to cause canting of the moments when they are in the plane
perpendicular to the c axis. The disappearance of the moment with a decrease in
temperature at 260 K is caused by a change in the anisotropy which causes the
moments to align along the c axis. In this configuration, spin canting does not
reduce the energy.
Hematite is part of a complex solid solution oxyhydroxide system having various
degrees of water, hydroxyl group, and vacancy substitutions that affect the
mineral's magnetic and crystal chemical properties. Two other end-members are
referred to as protohematite and hydrohematite.
Note for Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2.
This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned
it the nickname fool's gold due to its resemblance to gold. Pyrite is the most
common of the sulfide minerals. The name pyrite is derived from the Greek
πυρίτης (puritēs), “of fire” or "in fire”, from πύρ (pur), “fire”. This name is
likely due to the sparks that result when pyrite is struck against steel or
flint. This property made pyrite popular for use in early firearms such as the
wheellock.
This mineral occurs as isometric crystals that usually appear as cubes. The cube
faces may be striated (parallel lines on crystal surface or cleavage face) as a
result of alternation of the cube and pyritohedron faces. Pyrite also frequently
occurs as octahedral crystals and as pyritohedra (a dodecahedron with pentagonal
faces). It has a slightly uneven and conchoidal fracture, a hardness of 6–6.5,
and a specific gravity of 4.95–5.10. It is brittle and can be identified in the
field by the distinctive odor released when samples are pulverized.
Pyrite is usually found associated with other sulfides or oxides in quartz
veins, sedimentary rock, and metamorphic rock, as well as in coal beds, and as
the replacement mineral in fossils. Despite being nicknamed fool's gold, small
quantities of gold are sometimes found associated with pyrite. In fact, such
auriferous pyrite is a valuable ore of gold.
Pyrite is used commercially for the production of sulfur dioxide, for use in
such applications as the paper industry, and in the manufacture of sulfuric
acid, although such applications are declining in importance. Pyrites can show
negative resistance and have experimentally been used in oscillator circuits as
radio detectors.
Pyrite is often confused with the mineral marcasite, a mineral whose name is
derived from the Arabic word for pyrite, due to their similar characteristics.
Marcasite is a polymorph of pyrite, which means it has the same formula as
pyrite but a different structure and, therefore, different symmetry and crystal
shapes. The formal oxidation states are, however, the same as in pyrite because
again the sulfur atoms occur in persulfide-like pairs. Marcasite/pyrite is
probably the most famous polymorph pair next to the diamond/graphite pair.
Appearance is slightly more silver.
Marcasite is metastable relative to pyrite and will slowly be changed to pyrite
if heated or given enough time. Marcasite is relatively rare, but may be locally
abundant in some types of ore deposits, such as Mississippi Valley-type Pb-Zn
deposits. Marcasite appears to form only from aqueous solutions.
Pyrite is often used in jewellery such as necklaces and bracelets, but although
the two are similar, marcasite cannot be used in jewellery as it tends to
crumble into powder. Adding to the confusion between marcasite and pyrite is the
use of the word marcasite as a jewellery trade name. The term is applied to
small polished and faceted stones that are inlaid in sterling silver, but even
though they are called marcasite, they actually contain pyrite.
Note for Transmission Electron Microscopy
Transmission electron microscopy (TEM) is a microscopy technique whereby a beam
of electrons is transmitted through an ultra thin specimen, interacting with the
specimen as it passes through it. An image is formed from the electrons
transmitted through the specimen, magnified and focused by an objective lens and
appears on an imaging screen, a fluorescent screen in most TEMs, plus a monitor,
or on a layer of photographic film, or to be detected by a sensor such as a CCD
camera. The first practical transmission electron microscope was built by Albert
Prebus and James Hillier at the University of Toronto in 1938 using concepts
developed earlier by Max Knoll and Ernst Ruska.
The capabilities of the TEM can be further extended by additional stages and
detectors, sometimes incorporated on the same microscope. An electron
cryomicroscope is a TEM with a specimen holder capable of maintaining the
specimen at liquid nitrogen or liquid helium temperatures. This allows imaging
specimens prepared in vitreous ice, the preferred preparation technique for
imaging individual molecules or macromolecular assemblies.
A TEM can be modified into a scanning transmission electron microscope (STEM) by
the addition of a system that rasters the beam across the sample to form the
image, combined with suitable detectors.
An analytical TEM is one equipped with detectors that can determine the
elemental composition of the specimen by analysing its X-ray spectrum or the
energy-loss spectrum of the transmitted electrons.
Modern research TEMs may include aberration correctors, to reduce the amount of
distortion in the image, allowing information on features on the scale of 0.1 nm
to be obtained (resolutions down to 0.05 nm have been achieved) at
magnifications of 50 million times. Monochromators may also be used which reduce
the energy spread of the incident electron beam to less than 0.15 eV. Major TEM
makers include JEOL, Hitachi High-technologies, FEI Company (from merging with
Philips Electron Optics) and Carl Zeiss.
The TEM is used heavily in both material science/metallurgy and the biological
sciences. In both cases the specimens must be very thin and able to withstand
the high vacuum present inside the instrument.
For biological specimens, the maximum specimen thickness is roughly 1 micrometre.
To withstand the instrument vacuum, biological specimens are typically held at
liquid nitrogen temperatures after embedding in vitreous ice, or fixated using a
negative staining material such as uranyl acetate or by plastic embedding.
Typical biological applications include tomographic reconstructions of small
cells or thin sections of larger cells and 3-D reconstructions of individual
molecules via Single Particle Reconstruction.
In material science/metallurgy the specimens tend to be naturally resistant to
vacuum, but must be prepared as a thin foil, or etched so some portion of the
specimen is thin enough for the beam to penetrate. Preparation techniques to
obtain an electron transparent region include ion beam milling and wedge
polishing. The focused ion beam (FIB) is a relatively new technique to prepare
thin samples for TEM examination from larger specimens. Because the FIB can be
used to micro-machine samples very precisely, it is possible to mill very thin
membranes from a specific area of a sample, such as a semiconductor or metal.
Materials that have dimensions small enough to be electron transparent, such as
powders or nanotubes, can be quickly produced by the deposition of a dilute
sample containing the specimen onto support grids. The suspension is normally a
volatile solvent, such as ethanol, ensuring that the solvent rapidly evaporates
allowing a sample that can be rapidly analysed.
Note for Scanning Electron Microscope
The scanning electron microscope (SEM) is a type of electron microscope that
creates various images by focusing a high energy beam of electrons onto the
surface of a sample and detecting signals from the interaction of the incident
electrons with the sample's surface. The type of signals gathered in a SEM
varies and can include secondary electrons, characteristic x-rays, and back
scattered electrons. In a SEM, these signals come not only from the primary beam
impinging upon the sample, but from other interactions within the sample near
the surface. The SEM is capable of producing high-resolution images of a sample
surface in its primary use mode, secondary electron imaging. Due to the manner
in which this image is created, SEM images have great depth of field yielding a
characteristic three-dimensional appearance useful for understanding the surface
structure of a sample. This great depth of field and the wide range of
magnifications are the most familiar imaging mode for specimens in the SEM.
Characteristic x-rays are emitted when the primary beam causes the ejection of
inner shell electrons from the sample and are used to tell the elemental
composition of the sample. The back-scattered electrons emitted from the sample
may be used alone to form an image or in conjunction with the characteristic
x-rays as atomic number contrast clues to the elemental composition of the
sample.
In a typical SEM, electrons are thermionically emitted from a tungsten or
lanthanum hexaboride (LaB6) cathode and are accelerated towards an anode;
alternatively, electrons can be emitted via field emission (FE). Tungsten is
used because it has the highest melting point and lowest vapour pressure of all
metals, thereby allowing it to be heated for electron emission. The electron
beam, which typically has an energy ranging from a few hundred eV to 100 keV, is
focused by one or two condenser lenses into a beam with a very fine focal spot
sized 0.4 nm to 5 nm. The beam passes through pairs of scanning coils or pairs
of deflector plates in the electron optical column, typically in the objective
lens, which deflect the beam horizontally and vertically so that it scans in a
raster fashion over a rectangular area of the sample surface. When the primary
electron beam interacts with the sample, the electrons lose energy by repeated
scattering and absorption within a teardrop-shaped volume of the specimen known
as the interaction volume, which extends from less than 100 nm to around 5 µm
into the surface. The size of the interaction volume depends on the electrons'
landing energy, the atomic number of the specimen and the specimen's density.
The spatial resolution of the SEM depends on the size of the electron spot,
which in turn depends on both the wavelength of the electrons and the magnetic
electron-optical system which produces the scanning beam. The resolution is also
limited by the size of the interaction volume, or the extent to which the
material interacts with the electron beam. The spot size and the interaction
volume both might be large compared to the distances between atoms, so the
resolution of the SEM is not high enough to image individual atoms, as is
possible in the shorter wavelength (i.e. higher energy) transmission electron
microscope (TEM). The SEM has compensating advantages, though, including the
ability to image a comparatively large area of the specimen; the ability to
image bulk materials (not just thin films or foils); and the variety of
analytical modes available for measuring the composition and nature of the
specimen. Depending on the instrument, the resolution can fall somewhere between
less than 1 nm and 20 nm. In general, SEM images are easier to interpret than
TEM images.
This work was supported by the
Department of Energy's
Office of Basic Energy Sciences Geosciences Program.
The William R. Wiley Environmental Molecular Sciences
Laboratory is a national scientific user facility sponsored by the
Department of Energy's Office of Biological and Environmental Research and
located at Pacific Northwest
National Laboratory.
PNNL is a DOE Office of Science national laboratory that
solves complex problems in energy, national security and the environment, and
advances scientific frontiers in the chemical, biological, materials,
environmental and computational sciences. PNNL employs 4,000 staff, has a $760
million annual budget, and has been managed by Ohio-based Battelle since the
lab's inception in 1965.
In figure 1, Hematite in Scanning Electron Microscope ("SEM"),
magnification 20x
In figure 2, Pyrite
In figure 3, Iron building up in pyramids on one face of a hematite crystal
sends electrons to another face, which slowly dissolves
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